Neisseria meningitidis is a major cause of bacterial meningitis. All meningococcal strains have either a class 2 or class 3 major outer membrane protein of appropriately 35 to 40,000 Mr. These proteins are the meningococcal porins and carry the serotype determinants. They have been evaluated as vaccine constituents. Despite their common function, there is considerable antigenic diversity among the class 2 and 3 proteins. The purpose of the present studies is to determine the sequence of the class 3 protein and to compare it to the published sequence of the class 2 protein in an attempt to define the serotype specific regions. Since these proteins are produced in large quantities, the genes are very actively transcribed and thus have quite active promoters. Since synthesis of the foreign porin is lethal to E. coli, our strategy was to clone fragments of the class 3 gene. A 600 bp fragment believed to be from the gene for the class 3 protein was sequenced. To confirm this sequence we will compare it to the sequence of class-3 gene duplicated by PCR reaction using specific primers. Based on amino acid sequences deduced from DNA sequence, the structure of these proteins will be predicted. While constructing oligonucleotide probes to help define serotype specificity one of the probes was found to be unique for class-2. We are now attempting to find a probe unique to class-3.